The present invention relates constructing a hologram by optical interference of two coherent light beams and reconstructing the hologram.
A conventional method for constructing and reconstructing a hologram is illustrated in FIGS. 1 and 2. A plane wave light beam 2 and a spherical wave light beam 3 are irradiated onto a hologram record medium (photosensitive film) 1 such as silver halide or dichromated gelatin. These two hologram construction light beams 2 and 3 optically interfere with each other and interference fringe patterns 4 are formed on the photosensitive film 1. The hologram is recorded on the photosensitive film 1 by developing and fixing the interference fringes 5 are formed on the photosensitive film 1. An interference pattern is recorded on the beam 2 is a beam comprising rays of the same optical phase on each plane perpendicular to the propagation direction. The spherical wave light beam 3 is a beam radiated from a point or converging to a point and comprising rays of the same optical phase on a spherical surfaces. Interference fringes 5 are also formed in the sectional portion of the photosensitive film 1. The inclination angle of each interference fringe 5 depends upon the incidence angles of the hologram construction light beams 2 and 3.
When the hologram is to be reconstructed, a reconstruction plane wave light beam 6 is irradiated onto the hologram record in the direction opposite to that of the irradiation of the construction beam 2, as illustrated in FIG. 2. The reconstruction beam 6 is diffracted in the direction opposite to that of the other construction beam 3 at each point of the hologram, illustrated as a reconstructed beam 7.
Such a hologram is used, for example, as a point-of-sale (POS) scanner, as illustrated in FIG. 3. A laser beam 10 from a laser generating device 9 is irradiated onto a rotary hologram disc 8 comprising a transparent base plate upon which the holograms are fabricated. The diffracted reconstructed beam 10' irradiates and scans a barcode label 12 fixed to an article 11. The scanning beam 10' is back scattered from the barcode label 12 and a part of the scattered beam 13 passes through the hologram disc 8 and is again diffracted there. The diffracted beam 13 is detected by a detector 15 through a lens 14, thereby enabling the barcode number to be read.
In such a hologram scanner, the luminous intensity of the diffracted beam 13 must be high so as to upgrade the reliability of the detection of the barcode number. Therefore, the diffraction efficiency of the hologram, which is defined as the ratio of the diffracted output beam luminous intensity to the input beam luminous intensity, must be large.
Also, the diameter of the beam spot 16 (FIG. 4) irradiated by the scanning beam 10' on the barcode label 12 must be smaller than the gap between two adjacent bars 17, to obtain a signal 18 in response to the arrangement of the bars 17. If the beam spot is too large or deformed due to aberration of the beam, as illustrated in FIG. 5, the strength of the scanning signal 18a is reduced and the detection ability is degraded.
In the conventional hologram construction and reconstruction method, the hologram record medium swells or shrinks by chemical treatment during the developing process of the photosensitive film. FIG. 6(a) represents a conventional hologram construction process in which a plane wave light beam 2, which is irradiated perpendicularly to the photosensitive film 1, and a spherical wave light beam 3 having an incident angle of .alpha..sub.O at a point around the center of the photosensitive film 1, form interference fringes 5 on the film 1. When the photosensitive film 1 is developed, its thickness changes as illustrated in FIG. 6(b). Therefore, the inclination angle of each interference fringe 5 is changed. If a reconstruction beam 6 irradiates the shrunken hologram record medium (developed photosensitive film) la perpendicularly from the lower side of the hologram, the reconstructed beam 7 is diffracted in the direction opposite to that of the construction beam 3, since the interference fringe patterns on the hologram record medium surface do not change if the thickness of the hologram record medium changes. Regarding the reconstruction beam 6 and reconstructed beam 7, the inclination angle of each interference fringe 5 illustrated in FIG. 6(a) is an optimum angle, and thus the incidence angle of the reconstruction beam 6 is equal to the Bragg angle which is defined as the incidence angle of the reconstruction beam with respect to the hologram record medium surface, wherein the diffraction efficiency is maximized. Therefore, the inclination angle of each interference fringe 5 illustrated in FIG. 6(b) after the thickness change of the hologram record medium is no longer the optimum angle with result that the incidence angle of the reconstruction beam 6 is not equal to the Bragg angle. Accordingly, the diffraction efficiency is lowered so that the luminosity of the reconstructed beam is reduced.
In order to avoid such a degradation of the diffraction efficiency due to the change of the hologram record medium thickness, the off-set angle method is conventionally used. In this method, the incidence angles .alpha., .beta. of the construction beams 3, 2 are determined by considering the change of the inclination angle of the interference fringes, so that the changed inclination angle of each interference fringe nearly becomes the Bragg angle with respect to the reconstruction beam 6 and the reconstructed beam 7. However, the interference fringe patterns formed by this method are different from those of FIG. 6(a). Therefore, when the reconstruction beam 6 irradiates the hologram perpendicularly from below, as illustrated in FIG. 7(b), the aberration of the reconstructed beam 7 becomes large, which causes an obscure beam having a large diameter and deformation of the beam.
Another conventional method for compensating for the change of the inclination angle of the interference fringes is the hologram copying method. In this method, the interference fringes are copied, changing the inclination angle thereof without changing the patterns thereof, from a master hologram la having desired interference fringe patterns to another hologram record medium 1b, by irradiating a coherent plane wave light beam having an incidence angle of .theta. to the master hologram 1a, as illustrated in FIG. 8(a). A desired inclination angle of the interference fringes can be obtained by changing angle .theta.. However, in this method, unnecessary beams are reconstructed due to noise from unnecessary diffracted beams generated during the copying process, as illustrated in FIG. 8(b), so that the beam spot is obscured when used as a scanning beam and the S/N (signal/noise) ratio is degraded.
Also, in the conventional hologram construction and reconstruction method, when a beam having a wavelength of .lambda..sub.1 is to be used for reconstruction of the hologram and when the sensitivity of the photosensitive film is insufficient for the beam having a wavelength of .lambda..sub.1, construction beams having a wavelength of .lambda..sub.2, which can be sensed by the photosensitive film, must be used. In such a case, i.e., constructing the hologram by using beams having a wavelength of .lambda..sub.2, as illustrated in FIG. 9(a), and reconstructing the hologram by using a beam having a wavelength of .lambda..sub.1, as illustrated in FIG. 9(b), the reconstructed beam does not desirably converge at a required point and the aberration thereof is large.